The invention relates to a method and device for determining parameters of an autocorrelation function of an input signal V(t), the autocorrelation function being defined by the general formula: ##EQU2## and the form of the function .psi.(.tau.) being known. More particularly, the invention relates to the processing of electric or other signals in order to determine certain parameters of their autocorrelation function provided that the form of the function (e.g. an exponential form) is known in advance. The invention also relates to a device for performing the method and relates further to the application of the method and device to determining the size of particles in Brownian motion, e.g. particles suspended in a solvent, by a method of measurement based on analysis of fluctuations in the intensity of light diffused by the particles when they are illuminated by a ray of coherent light waves.
In the aforementioned method of determining the size of particles, it has already been proposed to determine the size of particles by a method in which an electric signal is derived corresponding to the fluctuations in the intensity of light diffused at a given angle, and the size of the particles is determined by analysis of the electric signal (B. Chu. Laser Light scattering, Annual Rev. Phys. Chem. 21 (1970) page 145 ff).
In order to analyze the electric signal it has already been proposed to use a wave analyzer to determine the size of the particles in dependence on the bandwidth of an average frequency spectrum of the electric signal. When a wave analyzer is used which operates on only one frequency at a time, by scanning, the aforementioned method has the serious disadvantage of requiring a good deal of time, so that not more than six or eight measurements can be made per day. If it is desired to reduce the measuring time by using a wave analyser which measures spectra over its entire width simultaneously, the disadvantage is that the apparatus becomes considerably more expensive, since such rapid analysers are complex and expensive.
In an improved method of analysing the electric signal, an autocorrelator for deriving a signal corresponding to the autocorrelation function of the electric signal is used together with a special computer connected to the autocorrelator output in order to derive a signal corresponding to the size of the particles by determining the time constant of the autocorrelation function, which is known to have a decreasing exponential form. This improved method can considerably reduce the measuring time compared with the method using a wave analyser, but it is still desirable to have a method and device which can determine the size of particles by less expensive and less bulky means. In this connection, it is noteworthy that commercial autocorrelators and special computers (for determining the time constant) are relatively expensive and bulky.
The previously-mentioned disadvantage, which was cited for a particular case, i.e. in determining the time constant of an exponential autocorrelation function, also affects the determination of other parameters of an autocorrelation function having a known form, e.g. linear or a Gaussian curve. As a rule, therefore, it is desirable to have a method and a device which can determine such parameters while avoiding the disadvantages mentioned hereinbefore in the case where the parameter to be determined is a time constant.